In a wireless communications system or cellular radio communications system wireless devices and/or user equipments, also known as mobile terminals and/or wireless terminals, communicate via a Radio Access Network (RAN) with one or more core networks. The user equipments may be mobile stations or user equipment units such as mobile telephones, also known as “cellular” telephones, and laptops with wireless capability, e.g., mobile termination, and may thus be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data via the radio access network. A wireless device may be any equipment being wirelessly connectable to a RAN for wireless communication.
The radio access network covers a geographical area which is divided into point coverage areas, traditionally denoted cells, with each point coverage area or cell being served by a base station, e.g., a Radio Base Station (RBS), which in some networks is also called “eNB”, “eNodeB”, “NodeB” or “B node” and which in this document also is referred to as a base station or radio network node. A point coverage area is a geographical area where radio coverage is provided by a point, also referred to as a “transmission point” and/or a “reception point”, which is controlled by the radio base station or radio network node at a base station site or radio network node site. A point coverage area is often also denoted a cell, but the concept of a cell also has architectural implications and the transmission of certain reference signals and system information. More specifically, multiple point coverage areas may jointly form a single logical cell sharing the same physical cell ID. However, in the following the notation of a “cell” is used interchangeably with “point coverage area” to have the meaning of the latter. Moreover, a point, or “transmission point” and/or a “reception point”, corresponds in the present disclosure to a set of antennas covering essentially the same geographical area in a similar manner. Thus, a point might correspond to one of the sectors at a site, e g a base station site, but it may also correspond to a site having one or more antennas all intending to cover a similar geographical area. Often, different points represent different sites. Antennas correspond to different points when they are sufficiently geographically separated and/or have antenna diagrams pointing in sufficiently different directions.
The radio network node communicates over an air interface or radio interface with the user equipments within the range of the radio network node. One radio network node may serve one or more cells via one or more antennas operating on radio frequencies. The cells may be overlaid on each other, e g as macro and pico cells having different coverage areas, or adjacent to each other, e g as so called sector cells where the cells served by the radio network node each cover a section of the total area or range covered by the radio network node. The cells adjacent or overlaid relative to each other may alternatively or additionally be served by different or separate radio network nodes that may be co-located or geographically separated.
The one or more antennas controlled by the radio network node may be located at the site of the radio network node or at antenna sites that may be geographically separated from each other and from the site of the radio network node. There may also be one or more antennas at each antenna site. The one or more antennas at an antenna site may be arranged as an antenna array covering the same geographical area or arranged so that different antennas at the antenna site have different geographical coverage. An antenna array may also be co-located at one antenna site with antennas that have different geographical coverage as compared to the antenna array. In the subsequent discussion an antenna or antenna array covering a certain geographical area is referred to as a point, or transmission and/or reception point, or more specifically for the context of this disclosure as a Transmission Point (TP). In this context multiple transmission points may share the same physical antenna elements, but could use different virtualizations, e.g., different beam directions.
The communications, i e transmission and reception of signals between the radio access network and a user equipment, may be performed over a communication link or communication channel via one or more transmission and/or reception points that may be controlled by the same or different radio network nodes. A signal may thus, for example, be transmitted from multiple antennas by being transmitted via one transmission point from more than one antenna in an antenna array or by being transmitted via more than one transmission point from one antenna at each transmission point. The coupling between a transmitted signal and a corresponding received signal over the communication link may be modelled as an effective channel comprising the radio propagation channel, antenna gains, and any possible antenna virtualizations. Antenna virtualization is obtained by precoding a signal so that it can be transmitted on multiple physical antennas, possibly with different gains and phases. Link adaptation may be used to adapt transmission and reception over the communication link to the radio propagation conditions.
An antenna port is a “virtual” antenna, which is defined by an antenna port-specific reference signal. An antenna port is defined such that the channel over which a symbol on the antenna port is conveyed can be inferred from the channel over which another symbol on the same antenna port is conveyed. The signal corresponding to an antenna port may possibly be transmitted by several physical antennas, which may also be geographically distributed. In other words, an antenna port may be virtualized over one or several transmission points. Conversely, one transmission point may transmit one or several antenna ports.
Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a multiple-input multiple-output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO.
The Long Term Evolution (LTE) standard is currently evolving with enhanced MIMO support. A core component in LTE is the support of MIMO antenna deployments and MIMO related techniques. A current working assumption in LTE-Advanced, i e 3GPP Release-10, is the support of an eight-layer spatial multiplexing mode with possibly channel dependent precoding. The spatial multiplexing mode is aimed for high data rates in favourable channel conditions. An illustration of the spatial multiplexing mode is provided in FIG. 1. Therein, the transmitted signal, represented by an information carrying symbol vector s is multiplied by an NT×r precoder matrix WNT×r, which serves to distribute the transmit energy in a subspace of the NT-dimensional vector space, corresponding to NT antenna ports. The precoder matrix is typically selected from a codebook of possible precoder matrices, and is typically indicated by means of a Precoder Matrix Indicator (PMI), which together with a Rank Indicator (RI) specifies a unique precoder matrix in the codebook. If the precoder matrix is confined to have orthonormal columns, then the design of the codebook of precoder matrices corresponds to a Grassmannian subspace packing problem. The r symbols in s each are part of a symbol stream, a so-called layer, and r is referred to as the rank or transmission rank. In this way, spatial multiplexing is achieved since multiple symbols can be transmitted simultaneously over the same Resource Element (RE) or Time-Frequency Resource Element (TFRE). The number of symbols r is typically adapted to suit the current channel properties.
LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink, and Discrete Fourier Transform (DFT) precoded OFDM in the uplink. The basic LTE physical resource can be seen as a time-frequency grid, as illustrated in FIG. 2, where each time-frequency resource element (TFRE) corresponds to one subcarrier during one OFDM symbol interval, on a particular antenna port. The resource allocation in LTE is described in terms of resource blocks, where a resource block corresponds to one slot in the time domain and 12 contiguous 15 kHz subcarriers in the frequency domain. Two time-consecutive resource blocks represent a resource block pair, which corresponds to the time interval upon which scheduling operates.
The received NR×1 vector yn for a certain resource element on subcarrier n or, worded differently, data RE number n or TFRE number n, assuming no inter-cell interference, is modeled byyn=HnWNT×rsn+en  (1)
where n denotes a transmission occasion in time and frequency, and en is a noise and interference vector obtained as realizations of a random process. The precoder, or precoder matrix, for rank r, WNT×r, can be a wideband precoder, which may be constant over frequency, or frequency selective.
The precoder matrix is often chosen to match the characteristics of the NR×NT MIMO channel Hn, also denoted channel matrix, resulting in so-called channel dependent precoding. When based on User Equipment (UE) feedback, this is also commonly referred to as closed-loop precoding and essentially strives for focusing the transmit energy into a subspace which is strong in the sense of conveying much of the transmitted energy to the UE or wireless device. In addition, the precoder matrix may also be selected to strive for orthogonalizing the channel, meaning that after proper linear equalization at the UE or wireless device, the inter-layer interference is reduced.
In closed-loop precoding, the UE or wireless device transmits, based on channel measurements in the forward link, i e the downlink, recommendations to the radio network node or base station of a suitable precoder to use. A single precoder that is supposed to cover a large bandwidth, so called wideband precoding, may be fed back. It may also be beneficial to match the frequency variations of the channel and instead feed back a frequency-selective precoding report, e.g. several precoders, one per subband. This is an example of the more general case of Channel State Information (CSI) feedback, which also encompasses feeding back other entities or information than precoders to assist the radio network node or base station in subsequent transmissions to the UE or wireless device. Such other information may include Channel Quality Indicators (CQIs) as well as Rank Indicator (RI).
In Release 8 and 9 of LTE the CSI feedback is given in terms of a transmission rank indicator (RI), a precoder matrix indicator (PMI), and channel quality indicator(s) (CQI). The CQI/RI/PMI report can be wideband or frequency selective depending on which reporting mode that is configured. This means that for CSI feedback LTE has adopted an implicit CSI mechanism where a UE does not explicitly report e.g., the complex valued elements of a measured effective channel, but rather the UE recommends a transmission configuration for the measured effective channel. The recommended transmission configuration thus implicitly gives information about the underlying channel state.
The RI corresponds to a recommended number of streams that are to be spatially multiplexed and thus transmitted in parallel over the effective channel. The PMI identifies a recommended precoder (in a codebook) for the transmission, which relates to the spatial characteristics of the effective channel. The CQI represents a recommended transport block size, i.e., code rate. There is thus a relation between a CQI and a Signal to Interference and Noise Ratio (SINR) of the spatial stream(s) over which the transport block is transmitted. Therefore, noise and interference estimates are important quantities when estimating, for example, the CQI, which is typically estimated by the UE or wireless device and used for link adaptation and scheduling decisions at the radio network node or base station side.
The term en in (1) represents noise and interference in a TFRE and is typically characterized in terms of second order statistics such as variance and correlation. The interference can be estimated in several ways. For example, estimates may be formed based on TFREs containing cell specific RS since sn and WNT×r are then known and Hn is given by the channel estimator. The interference may then be estimated as the residual noise and interference on the TFREs of the Cell Specific Reference Signal (CRS), after the known CRS sequence has been pre-subtracted, i.e., after the CRS has been cancelled. An illustration of CRS, sometimes read out as Cell-specific Reference Symbols, for Rel-8 of LTE can be seen in FIG. 3. It is further noted that the interference on TFREs with data that is scheduled for the UE in question can also be estimated as soon as the data symbols, sn are detected, since at that moment they can be regarded as known symbols. The latter interference can alternatively also be estimated based on second order statistics of the received signal and the signal intended for the UE of interest, thus possibly avoiding needing to decode the transmission before estimating the interference term. Alternatively the interference can be measured on TFREs where the desired signal, i e the signal intended for the UE of interest, is muted, so the received signal corresponds to interference only. This has the advantage that the interference measurement may be more accurate and the UE processing becomes trivial because no decoding or desired signal subtraction need to be performed.
In LTE Release-10, a new reference symbol sequence was introduced, the Channel State Information Reference Signal (CSI-RS), intended to be used for estimating channel state information. The CSI-RS provides several advantages over basing the CSI feedback on the CRS which were used, for that purpose, in previous releases. Firstly, the CSI-RS is not used for demodulation of the data signal, and thus does not require the same density. This means that the overhead of the CSI-RS is substantially less as compared to that of CRS. Secondly, CSI-RS provides a much more flexible means to configure CSI feedback measurements: For example, which CSI-RS resource to measure on can be configured in a UE specific manner. Moreover, antenna configurations larger than 4 antennas must resort to CSI-RS for channel measurements, since the CRS is only defined for at most 4 antennas.
A detailed example showing which resource elements within a resource block pair may potentially be occupied by UE-specific RS and CSI-RS is provided in FIG. 4. In this example, the CSI-RS utilizes an orthogonal cover code of length two to overlay two antenna ports on two consecutive REs. As seen, many different CSI-RS patterns are available. For the case of 2 CSI-RS antenna ports, for example, there are 20 different patterns within a subframe. The corresponding number of patterns is 10 and 5 for 4 and 8 CSI-RS antenna ports, respectively.
A CSI-RS resource may be described as the pattern of resource elements on which a particular CSI-RS configuration is transmitted. One way of determining a CSI-RS resource is by a combination of the parameters “resourceConfig”, “subframeConfig”, and “antennaPortsCount”, which may be configured by Radio Resource Control (RRC) signaling.
Based on a specified CSI-RS resource, that defines an effective channel for the data transmission, and an interference measurement configuration, e.g. a muted CSI-RS resource, the UE can estimate the effective channel and noise plus interference, and consequently also determine which rank, precoder and transport format to recommend that best match the particular effective channel.
Furthermore, in order to improve system performance, for example by improving the coverage of high data rates, improving the cell-edge throughput and/or increasing system throughput, Coordinated Multipoint (CoMP) transmission and/or reception may be used in a wireless communications system or radio access network. In particular, the goal is to distribute the user perceived performance more evenly in the network by taking control of the interference in the system, either by reducing the interference and/or by better prediction of the interference. To harvest the gains of introducing coordinated transmission or CoMP feedback it is essential that a radio network node or base station, e g an eNodeB, can accurately predict the performance of a UE or wireless device for various coordinated transmission hypotheses, in order to select an appropriate downlink assignment. To this end, accurate interference measurements at a terminal are a key element for CSI reporting targeting different transmission hypotheses. However, current state of the art solutions for interference measurements are constrained by current standards and/or limitations imposed by UE specific muting of data channels, making accurate interference measurements difficult, in particular for CoMP systems employing dynamic point selection and/or joint transmission, where the transmission point association to a UE varies dynamically in time.
Moreover, it is often beneficial for a scheduler in a radio network node or base station such as an eNodeB to receive CSI reports that are based on a predictable and robust interference level. When a UE or wireless device measures interference caused by other data transmissions, the measured interference level will vary with the current traffic load and moreover will see rapid power variations due to the so called flash-light effect where dynamic precoding and/or beamforming in interfering points cause rapid and often unpredictable interference variations. For CSI reporting, such variations typically degrade the overall performance, since the measured interference often underestimates the interference seen at the subsequent data transmission allocation that is based on the CSI report. As a consequence, the radio network node or base station may have to reduce the data rate in the link adaptation to avoid excessive retransmissions due to uncertainties in the reported CQI.
To improve the possibilities for UEs to perform accurate interference measurements in a system zero-power (ZP) CSI-RS resources, also known as a muted CSI-RS have been introduced. The zero-power CSI-RS resources are configured just as regular CSI-RS resources, so that a UE knows that the data transmission is mapped around those resources. The intent of the zero-power CSI-RS resources is to enable the network to mute the transmission on the corresponding resources so as to boost the SINR of a corresponding non-zero power CSI-RS, possibly transmitted in a neighbor cell/transmission point. For Rel-11 of LTE, a special zero-power CSI-RS that a UE is mandated to use for measuring interference plus noise is under discussion. As the name indicates, a UE can assume that the TPs of interest are not transmitting on the muted CSI-RS resource and the received power can therefore be used as a measure of the interference plus noise level. For the purpose of improved interference measurements the agreement in LTE Release 11 is that the network will be able to configure a UE to measure interference on a particular Interference Measurement Resource (IMR) that identifies a particular set of TFREs that is to be used for a corresponding interference measurement.
However, reserving resources for specific purposes such as interference measurements reduces the resources available for data transmission. In some system configurations such reserved resources may give rise to a significant overhead, e g in the downlink when used to enable UEs to perform more reliable interference measurements.
Thus, there is a need for improving the efficiency in use of radio resources for reliably determining the interference that can be expected when receiving a signal over a communication channel from a radio access network.